When fetching data from the database as assoc, you will get an associative array (which when converted to json will become a json object) as result.
What you are currently doing is adding the row (which is an associative array with one key-value) to your result array. If you only want the city, you can easily fetch only that data from the assoc-array.
Changing

$restaurantArray[] = $rows;

to

$resturantArray[] = $rows['city'];

should do the trick.

Additionally, if you wanted to reformat the data in the result, and not just push the city name to it, a array_map call could be used: